Daegu FC manager Byun Byung-joo brought the fast and extremely aggressive "bullet football" tactic to his team.

[12] The next year, he was transferred to Sangju Sangmu to perform his military service as per South Korean law.

He was selected for the 2007 AFC Asian Cup, and also capped for the South Korea under-23 team in qualification for the 2008 Summer Olympics.

[13] On 15 October 2008, Lee scored two goals in a qualification match of the 2010 FIFA World Cup against United Arab Emirates.

Lee was included in South Korea's squad for the 2014 FIFA World Cup by showing good performances with three goals in the final round of the qualification.

In their first group stage match against Russia on 17 June 2014, he came on as a substitute for Park Chu-young in the 52nd minute and scored his first ever World Cup goal in a 1–1 draw when his shot from outside the box was spilled by goalkeeper Igor Akinfeev over his head and over the line.

[17][18] In 2015, he was appointed as the promotional ambassador of the Purme Foundation, which helps disabled children in their path of rehabilitation and is based in Gangneung.

[19][20] During his fellowship with the association, he organized several charity football matches and clinics to help the child patients by providing them with scholarships and appropriate equipment.

[20] Following the forest wildfire that brought severe damages all over the Gangwon Province in 2019,[21][22] Lee offered financial support to help the process of recover and rebuilding.